We simulate lattice QCD with a finite chemical potential for isospin and 2-colour QCD with a finite chemical potential for quark number. For large chemical potentials both these theories have a superfluid phase which is the analogue of the colour superconducting phase expected for QCD at large baryon-number chemical potential. We measure the dependence of the lattice QCD finite temperature transition on the isospin chemical potential for small chemical potential where the dependence on isospin and quark-number chemical potentials is expected to be identical, and at large chemical potentials where the pion condensate evaporates.